The present invention relates to the manufacture of photomasks and phase-shift masks. In particular, the invention relates to methods for forming chrome photomasks and for forming phase-shift masks without producing chrome opaque defects.
A conventional photomask comprises a patterned light-shielding film of opaque material, typically a metal such as chromium, on a transparent mask substrate, typically silica (quartz). In photomask manufacture, a photoresist is applied to the opaque-material side of a mask blank comprising a layer of the opaque material on a transparent mask substrate. The photoresist is patterned by an image-wise exposure and wet developed to produce a pattern of photoresist over the opaque layer. The mask blank containing the imaged photoresist is either wet or dry etched to remove the opaque material revealed by removal of the photoresist. When the photoresist is stripped after etching, a patterned layer of opaque material remains on the transparent substrate.
Typically, a positive-tone, novolac-type photoresist is exposed by a laser tool and developed with TMAH: a dilute solution of tetramethyl ammonium hydroxide. TMAH developer is a strong base. The exposed portions of the opaque layer are wet etched with a ceric salt, typically ceric ammonium nitrate in 10% nitric acid or a ceric salt in 10% perchloric acid. An acidic solution of a ceric salt is a strong oxidizing agent.
When a wet etch is used, it is common practice to perform both the develop step and wet etch step sequentially in the same process chamber of the process tool. These steps are typically carried out using a spray process in which the developer and wet etchant are sequentially sprayed onto a rotating mask substrate in a single process chamber. The usual sequence is: develop, rinse with deionized water, wet etch, rinse with deionized water, and dry.
The ceric ion is soluble in the acidic etch solution. Basic ceric salts precipitate in neutral solution or in the basic developer solution, however, and form small particles across the entire mask blank. Over time, a steady build up of yellow ceric hydroxide particles, orange ceric ammonium nitrate particles, and mixtures of these cerium-containing particles forms and deposits throughout the process chamber. The nozzles that spray the rinse water also become contaminated with cerium. The rinse water contains cerium in both soluble and insoluble forms.
If a dry etch is used in place of a wet etch, the mask blank containing the patterned photoresist is developed with TMAH, washed with deionized water, dried, and transferred to a reactive ion etch system for the dry etch of the chrome layer. Although the dry etch does not use a cerium etchant, the process tool used to develop the photoresist is typically contaminated with cerium from other processing. Consequently, the mask blank is contaminated with cerium-containing particles during development. These particles are not removed by the deionized water rinse; rather, they produce chromium opaque defects on the resulting photomask due to micro-masking by the cerium-containing particles during dry etch of the chromium layer.
Most process tool manufacturers recommend a once-a-month manual washdown of the process tool with concentrated hydrochloric acid to remove the cerium compounds from the process tool. In addition to causing downtime, manual washdown is time consuming and inconvenient. Concentrated hydrochloric acid is corrosive and the precipitated cerium compounds and hydrochloric acid can react to produce chlorine gas and nitrogen dioxide.
Thus, a need exists for methods for removing precipitated particles of ceric salts from mask blanks prior to wet or dry etching to form photomasks and for methods for removing cerium deposits from process tools.
To meet this and other needs, and in view of its purposes, the present invention provides a method for forming a photomask on a mask blank having a transparent substrate and a chrome layer. When a dry etch is used to etch the chrome layer, the method comprises, in order, the steps of:
a) applying photoresist to the chrome layer of the mask blank, the mask blank either (i) having the chrome layer on the transparent substrate, or (ii) comprising, in order, the chrome layer, a phase-shift mask layer, and the transparent substrate;
b) patterning the photoresist;
c) wet developing the photoresist and removing a portion of the photoresist, revealing a portion of the underlying chrome layer;
d) rinsing the mask blank with water;
e) rinsing the mask blank with dilute acid;
f) rinsing the mask blank with water;
g) drying the mask blank;
h) dry etching the chrome layer and removing the portion of the chrome layer revealed in step c); and
i) stripping the photoresist from the mask blank.
When a wet etch is used to etch the chrome layer, the method comprises, in order, the steps of:
a) applying photoresist to the chrome layer of the mask blank, the mask blank either (i) having the chrome layer on the transparent substrate, or (ii) comprising, in order, the chrome layer, a phase-shift mask layer, and the transparent substrate;
b) patterning the photoresist;
c) wet developing the photoresist and removing a portion of the photoresist, revealing a portion of the underlying chrome layer;
d) rinsing the mask blank with water;
e) wet etching the chrome layer with an acidic solution of a ceric salt and removing the portion of the chrome layer revealed in step c);
f) rinsing the mask blank with water;
g) rinsing the mask blank with dilute acid;
h) rinsing the mask blank with water; and
i) stripping the photoresist from the mask blank.
The dilute acid does not attack the transparent substrate, the chrome layer, the phase-shift mask layer, or the photoresist, and cerium salts are soluble in the dilute acid. The dilute acid is preferably nitric acid or perchloric acid, more preferably nitric acid. The transparent substrate is preferably silica.
This method decreases the number of defects per photomask as well as the mask-to-mask variation in the number of defects. In addition, the acid rinse does not affect the photoresist so it is unnecessary to modify or change any other part of the overall process. The acid spray also prevents or removes the buildup of cerium-containing deposits in the process tool. This advantage reduces the number of times that the process tool must be cleaned to remove the buildup of cerium-containing deposits.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.